The condition is called "commotio cordis", and was first documented in 1876.

It is extremely rare - but a study in the US found 128 cases, most involving strikes by baseballs or softballs.

There have been cases involving cricket balls or footballs - and some were caused by corporal punishment, play-fighting or even a snowball fight.

One documented case in a child was caused by the family dog jumping into the patient's chest.

Dr Mark Link, from the Tufts-New England Medical Center in Boston is researching the subject by firing balls at the chests of pigs, with the shots timed to coincide exactly with specific moments in the heartbeat cycle of the animal.

He concluded that the risk is greatest if the blow happens during a 15 millisecond window during a heartbeat, and the ball strikes directly over the heart.

The research suggests that the blow causes a "spike" in blood pressure which stretches the heart muscle.

This starts a chemical chain reaction which causes the organ to contract far too early, stopping it mid-cycle.

No magic

Dr Link believes that anyone is vulnerable to commotio cordis provided they are struck in exactly the same way.

In addition, fitting the pigs with chest protectors similar to those worn in ice hockey, lacrosse or baseball offered no protection whatsoever.

Dr Peter Kohl, a British expert who has carried out research into commotio cordis, said: "Commotio coris has the air of a hard-to-understand, black magic thing - but it isn't."